A plastic foam is used for various applications because it exerts heat-shielding properties, heat-insulating properties, sound-shielding properties, sound-absorbing properties, damping properties, reduction in weight, and the like, according to the material of the foam and the state of formed bubbles. Such a plastic foam may be produced by a method in which a masterbatch containing a chemical foaming agent is foamed with heating to mold the plastic foam. However, handling thereof was problematically difficult, for example, in that the masterbatch containing a chemical foaming agent may not foam even after being heated, and the foaming agent may decompose rapidly in an injection foam-molding machine. Moreover, since some resins fail to provide a sufficient expansion ratio, a desired hardness as a molding product may be less likely to be obtained.
Patent Document 1 discloses that an injection foamed product having a high hardness and expansion ratio and containing uniform bubbles is obtained from any resin by using a masterbatch pellet of an ethylene-α-olefin copolymer containing a chemical foaming agent.
However, chemical foaming agents decomposed by heating simultaneously generate decomposed gas and a foaming residue, and the residue remaining in the molding product sometimes affects the adhesion performance of the molding product. Moreover, upon use of chemical foaming agents, not all of the bubbles of a foamed product serve as closed cells but some of the bubbles inevitably serve as open cells. A foamed product having high airtightness is, problematically, less likely to be obtained.
Patent Document 2 discloses a foamed resin masterbatch in which polyolefin resin or styrene resin is used as a base resin, and a thermally expandable microcapsule, instead of a chemical foaming agent, is used as a foaming agent.
However, upon use of the thermally expandable microcapsule described in Patent Document 2, the expansion ratio of the foam to be obtained is low, resulting in difficulty in forming closed cells of the foam to be obtained at a certain size.
Patent Document 3 discloses a method for producing a foam composite panel by foaming and molding a resin composition obtained by blending a masterbatch containing a thermally expandable microcapsule with a masterbatch containing a chemical foaming agent.
However, even in the case of such a method, the expansion ratio of the molding product is low, and performances, such as a desired lightness and heat insulation, are not obtained although the expansion ratio is somewhat improved. Moreover, it is difficult to obtain a molding product having a good appearance.
Patent Document 4 discloses a synthetic resin composition containing a thermally expandable microcapsule and a base resin, and a production method thereof. Such a synthetic resin composition contains a base resin having a melt flow rate within a predetermined range. Therefore, the thermally expandable microcapsule is excellently miscible and compatible with the base resin without breaking the shell of the thermally expandable microcapsule.
Patent Document 1: Japanese Kokai Publication 2000-178372 (JP-A 2000-178372);
Patent Document 2: Japanese Kokai Publication Hei-11-343362 (JP-A Hei-11-343362);
Patent Document 3: Japanese Kokai Publication 2005-212377 (JP-A 2005-212377);
Patent Document 4: Japanese Kokai Publication 2002-264173 (JP-A 2002-264173)